fix(tools): use start_new_session instead of preexec_fn to prevent SIGSEGV in multi-threaded processes

preexec_fn=os.setsid runs Python code in the forked child before exec,
which is unsafe in multi-threaded processes (CPython docs). When the
Desktop gateway loads native libraries (onnxruntime, BLAS, provider SDKs)
with active thread pools, the fork can SIGSEGV before the child execs.

Replace all preexec_fn usage with start_new_session=True, which provides
the same setsid/process-group semantics without running Python in the
fork. This is already the pattern used throughout hermes_cli/gateway.py
and hermes_cli/_subprocess_compat.py.

Fixes #46789

plugins/platforms/whatsapp/adapter.py

@@ -599,7 +599,7 @@ class WhatsAppAdapter(WhatsAppBehaviorMixin, BasePlatformAdapter):
                 ],
                 stdout=bridge_log_fh,
                 stderr=bridge_log_fh,
-                preexec_fn=None if _IS_WINDOWS else os.setsid,
+                start_new_session=True,
                 env=bridge_env,
             )
             _write_bridge_pidfile(self._session_path, self._bridge_process.pid)

tests/tools/test_windows_compat.py

@@ -46,6 +46,25 @@ class TestNoUnconditionalSetsid:
             )
 
 
+class TestStartNewSession:
+    """All guarded files must use start_new_session=True instead of preexec_fn."""
+
+    @pytest.mark.parametrize("relpath", GUARDED_FILES)
+    def test_uses_start_new_session(self, relpath):
+        """Each guarded file must use start_new_session=True for process isolation."""
+        filepath = PROJECT_ROOT / relpath
+        if not filepath.exists():
+            pytest.skip(f"{relpath} not found")
+        source = filepath.read_text(encoding="utf-8")
+        # Files should use start_new_session=True, not preexec_fn
+        assert "preexec_fn" not in source, (
+            f"{relpath} still uses preexec_fn; use start_new_session=True instead"
+        )
+        assert "start_new_session=True" in source, (
+            f"{relpath} missing start_new_session=True in Popen call"
+        )
+
+
 class TestIsWindowsConstant:
     """Each guarded file must define _IS_WINDOWS."""
 

tools/code_execution_tool.py

@@ -1290,7 +1290,7 @@ def execute_code(
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
             stdin=subprocess.DEVNULL,
-            preexec_fn=None if _IS_WINDOWS else os.setsid,
+            start_new_session=True,
             creationflags=subprocess.CREATE_NO_WINDOW if _IS_WINDOWS else 0,
         )
 

tools/environments/local.py

@@ -731,7 +731,7 @@ class LocalEnvironment(BaseEnvironment):
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT,
             stdin=subprocess.PIPE if stdin_data is not None else subprocess.DEVNULL,
-            preexec_fn=None if _IS_WINDOWS else os.setsid,
+            start_new_session=True,
             cwd=_popen_cwd,
             **_popen_kwargs,
         )

tools/process_registry.py

@@ -750,7 +750,7 @@ class ProcessRegistry:
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT,
             stdin=subprocess.DEVNULL,
-            preexec_fn=None if _IS_WINDOWS else os.setsid,
+            start_new_session=True,
             **_popen_kwargs,
         )